A scanning electron microscope (hereinafter shorted as SEM in this specification) obtains a magnified 2-dimensional scanning image of an object to be examined by emitting a beam of electrons from a heating type or a field emission type, concentrating it by an electrostatic or magnetic lens into a fine electron beam (a primary electron beam), applying the electron beam onto the object to be examined in a 2-dimensional scanning manner, detecting a secondary signal electrons that are secondarily generated or reflected from the object, and feeding the magnitude of the detected signal to the brightness modulation of a CRT tube which is scanned in synchronism with the primary electron beam.
A conventional SEM emits electrons from an electron source to which a negative voltage is applied, accelerates them by anodes of a grounding voltage, and applies the accelerated electrons (a primary electron beam) to an object to be examined at a grounding voltage.
Recently, semiconductor chips have become smaller and their circuit patterns have become extremely fine. Accordingly, SEMs have been widely used in place of optical microscopes to inspect manufacturing processes of semiconductor chips and processed semiconductor chips (e.g. by measurement of dimensions by electron beams and inspection of electric operations),
Semiconductor specimens to be examined by the SEM are generally made of a multiple layers of electrically-insulating materials on a conductor such as aluminum or silicon. When an electron beam is applied to such a specimen, the surface of the specimen is charged up, which will change the direction of motion of the emitted secondary electrons or the primary electrons themselves. Consequently, the resulting images may have an extraordinary contrast, distortion, or the like. To reduce the influence by this charging up, the energy of the emitted electron beam must be made as low as possible.
However, if the energy of the emitted electron beam (an acceleration voltage) is made low, a chromatic aberration due to energy dispersion of the electron beam generates and the resolving power of the SEM drastically goes down, which makes observation at a high magnification harder.
As a means for solving such a problem, a technology for forming a deceleration field for electron beams (hereinafter called “retarding” in this specification) has been disclosed in Japanese Non-examined Patent Publication No.06-139985 (1994).
“Retarding” is a technology of forming a deceleration field by increasing the voltage to accelerate the electron beam to the anodes and applying a negative potential to the object to be examined, finally setting the acceleration voltage to a comparatively low level, and thus preventing chromatic aberration and charging up.